Electrophotographic sheet material generally comprises a base sheet, for example paper, having thereon a light-sensitive coating, e.g. one containing a photoconductive particulate material in an insulating binder. Typical of such coatings are those containing zinc oxide in a suitable resin binder, such as polyvinylacetate or modified polyvinylacetate. An electrostatic image may be formed on such sheet by exposure to light projected from an original, and a visible image may then be developed and fixed. The sheet with the visible image so formed may then in turn serve as a master plate for offset printing. When used in such capacity, the sheet is treated with an aqueous solution designed to render the non-imaged areas of the sheet hydrophilic (or oleophobic) before running it on the offset press. Conventional sheets tend to be deleteriously affected by such treatment, as could only be expected, notably in that they quickly stretch and crease due to contact with the fountain solution on the press, become weak or delaminate and so on. This limits the number of copies that can be printed with a single conventional master and with a simple conventional master it would be rare to print more than about 1000 copies.
Improvements have been proposed to make such offset printing masters more durable. Canadian Pat. No. 874,905 proposes to apply on a paper base a sizing layer containing a major amount of dialdehyde starch in addition to the usual sizing agents, such as carboxymethyl cellulose, gelatin and the like, and to apply the photosensitive coating on said sizing layers.
Canadian Pat. No. 862,679 proposes an electrophotographic material having an electroconductive back layer, e.g. of a cationic latex and an interlayer made of vinyl polymer and amino resins. These expedients however are either not particularly effective or are very cumbersome and expensive. For example, applying the vinyl polymer and amino resins to paper and curing at a temperature of about 150.degree. C. represents a complex and delicate task. Yet another proposal (Canadian Pat. No. 957,540) is to apply a resin coating, more particularly a cellulose ester coating, to the back of the paper but the results permit only up to 300 copies to be printed.